AKMD - two ways - 1. you dial a special 'Minicom' or textphone number and when it rings on another textphone then someone answers and you get to type at each other with your words appearing on a screen. In reality except for certain huge organisations, the textphone has been shoved in a drawer or no-one knows what to do and it doesn't work.
So what I usually use is what used to be called TypeTalk and is now TextDirect - you dial a prefix and then your number, and on a good day your screen displays 'Operator connected... dialling... ring... ring... explaining TypeTalk...GA' (go ahead), so I press a button, talk at the person, say Go Ahead when I'm done, and then whatever the person says gets typed by the operator and appears on my screen. If I couldn't talk then I'd be typing as well and the op reading it out, which would slow everything down further.
When it works, it works great, only slightly stilted. But an awful lot of the time the op gets as far as explaining they will be relaying what you say to a deaf person and they hang up, or even before the op gets to speak as BT have decided to insert a recorded message saying something like "please hold the line for a TextDirect call", and of course most people hear a recording and hang up. My record was over 50 hangups and people insulting the operator... when I was trying to book a hearing test for my son - my HV ensured that clinic got bollocked beyond belief!
The system also runs on ancient analogue modem technology so can miss bits. Ideally we'd get with the 21st century and be able to integrate phones with instant messenger so I could just dial someone via my laptop (there's no mobile version...), but the various pilots of that haven't got far yet. There were a couple private companies providing a similar service that you could use for meetings and stuff but they were both shit and soon went bankrupt.
My pet peeve is forms requiring phone details with no space to put the text prefix in (18002) or to say 'do not phone me - text this no. or preferably email this address'. Luckily I can use an amplified phone to make a short call if really necessary and usually figure out I can have a GP appt at whatever time I repeat until they confirm I've understood, and most stuff I can do online, but anything involving pregnancy and children seems to assume you have a phone, so I just give out MrNC's details and he contacts me as need be.
Actually my real pet peeve is people who think that because I can speak beautiful RP English, I must actually be able to hear them, or the fact that I can hear fire alarms/traffic noise means I can understand speech. Yes, I can understand people, mainly men, with southernUK accents who I know who are in a quiet place who are saying stuff I'm expecting, when I have my hearing aids on and I'm not tired or in pain. If you're a female Australian in a noisy call centre not telling me what I'm expecting to hear, or I'm in labour, then it's terp time. And no, I don't understand BSL more than GCSE level, so don't get a BSL interpreter without asking me first - I need speech-to-text relay (basically a woman with a laptop typing!) Or just poor old MrNC again...
[and breathe...]